The present invention relates generally to techniques for forming substrates using a layer transfer technique and a shaped ingot. More particularly, the present method and system provides a layer transfer process for slicing a single crystal silicon sheet from a shaped silicon ingot for a variety of applications including photovoltaic cells. Merely by example, the present invention provides a method of using preferential cleave planes in single crystal silicon with a patterned implant to produce single crystal silicon sheets in a highly efficient controlled cleaving process. But it will be recognized that the invention has a wider range of applicability.
From the beginning of time, human beings have relied upon the “sun” to derive almost all useful forms of energy. Such energy comes from petroleum, radiant, wood, and various forms of thermal energy. As merely an example, human beings have relied heavily upon petroleum sources such as coal and gas for much of their needs. Unfortunately, such petroleum sources have become depleted and have led to other problems. As a replacement, in part, solar energy has been proposed to reduce our reliance on petroleum sources. As merely an example, solar energy can be derived from “solar cells” commonly made of silicon.
The silicon solar cell generates electrical power when exposed to solar radiation from the sun. The radiation interacts with atoms of the silicon and forms electrons and holes that migrate to p-doped and n-doped regions in the silicon body and create voltage differentials and an electric current between the doped regions. Solar cells have been integrated with concentrating elements to improve efficiency. As an example, solar radiation accumulates and focuses using concentrating elements that direct such radiation to one or more portions of active photovoltaic materials. Although effective, these solar cells still have many limitations.
As merely an example, solar cells often rely upon starting materials such as silicon. Such silicon is often made using either polysilicon (i.e. polycrystalline silicon) and/or single crystal silicon materials. These materials are often difficult to manufacture. Polysilicon cells are often formed by manufacturing polysilicon plates. Although these plates may be formed effectively in a cost effective manner, they do not possess optimum properties for highly effective solar cells. In particular, polysilicon plates do not exhibit the highest possible efficiency in capturing solar energy and converting the captured solar energy into usable electrical power. By contrast, single crystal silicon (c-Si) has suitable properties for high grade solar cells. Such single crystal silicon is, however, expensive to manufacture and is also difficult to use for solar applications in an efficient and cost effective manner.
Additionally, both polysilicon and single-crystal silicon materials suffer from material losses during conventional manufacturing single crystal silicon substrates, where a sawing process is used to physically separate thin single crystal silicon layers from a single crystal silicon ingot originally grown. For example, inner diameter (ID) sawing process or wire sawing process eliminates as much as 40% and even up to 60% of the starting material from a cast or grown boule and singulate the material into a wafer form factor. This is a highly inefficient method of preparing thin polysilicon or single-crystal silicon plates for solar cell use.
To overcome drawbacks of using silicon materials, thin-film solar cells have been proposed. Thin film solar cells are often less expensive by using less silicon material or alternative materials but their amorphous or polycrystalline structure are less efficient than the more expensive bulk silicon cells made from single-crystal silicon substrates. These and other limitations can be found throughout the present specification and more particularly below.
From the above, it is seen that techniques to manufacture suitable high quality single crystal silicon sheets with low cost and high productivity are highly desired.